1. Field of the Invention
The present invention relates to an image output apparatus, a control method therefor, an image display apparatus, a control method therefor, and a storage medium.
2. Description of the Related Art
Conventionally, a liquid crystal image display apparatus displays an image by keeping an amount of light of a backlight constant, and controlling a voltage applied to a liquid crystal layer and changing the transmittance of liquid crystal to adjust an amount of light transmitted through the liquid crystal layer. However, there is a limit in reducing the transmittance of the liquid crystal. Therefore, even if the transmittance of the liquid crystal is minimized, in some cases, the light of the backlight is transmitted through the liquid crystal layer and sufficient contrast is not obtained. As a countermeasure against this problem, there is known a technique for improving contrast by performing dimming control for adjusting an amount of light of a backlight according to an image signal.
The dimming control include global dimming for uniformly changing an amount of light of the entire backlight and local dimming for dividing the backlight into several control regions and independently changing an amount of light for each of the divided control regions. In the global dimming control, the number of light emission control units is one. In the local dimming control, the number of light emission control units is two or more.
In the dimming control, for example, for each of the control regions, a peak signal level of image data input to a display region of a liquid crystal panel corresponding to the control region is acquired. An amount of light of the backlight in the control region is adjusted to set the brightness of a display screen at the maximum transmittance of liquid crystal to brightness corresponding to the peak signal level. Again or a lookup table (LUT) is adjusted to set the peak signal level to a maximum signal level (see, for example, Japanese Patent Application Laid-Open No. H11-65531). The peak signal level is a highest signal level among signal levels of all pixels of the image data input to the display region of the liquid crystal panel corresponding to the control region. The LUT is a table in which signal levels to be output corresponding to signal levels to be input are set by an image processing unit of an image display apparatus. The liquid crystal panel is driven on the basis of image data subjected to image processing according to the LUT.
In recent years, a liquid crystal image display apparatus is used for, for example, diagnosis of a modality image in the medical field. The modality image is an image captured by a medical capturing apparatus for digital radiography (DR) or computed tomography (CT). In medical diagnosis, for example, two liquid crystal image display apparatuses are set side by side and caused to respectively display different images and comparative diagnosis is performed. For example, a modality image of a region captured this time is displayed on one image display apparatus and a modality image of the same capturing region and of the same kind captured in the past is displayed on the other image display apparatus. In the diagnosis based on the modality images, it is important that a human observer can recognize a brightness difference between gradations. Therefore, the image display apparatuses that display the modality images are required to have a high contrast characteristic and gradation reproducibility.
There is a technique for, in order to equalize dynamic ranges of a plurality of image display apparatuses having different displayable brightness ranges, setting a highest value among lowest brightnesses of the image display apparatuses as common lowest brightness and setting a lowest value among highest brightnesses of the image display apparatuses as common highest brightness (see, for example, Japanese Patent Application Laid-Open No. 2005-070080).
The dimming control is performed according to image data input to an image display apparatus. Therefore, when different image data are respectively input to a plurality of image display apparatuses, amounts of light of backlights determined by the dimming control could not be the same in the plurality of image display apparatuses. Therefore, in a form of use for causing the plurality of image display apparatuses to display different images and performing comparative diagnosis, amounts of light of the backlights could not be the same in the plurality of image display apparatuses.
Then, gradation reproducibility such as a degree of floating black is different among the image display apparatuses. Therefore, it is likely that accurate image comparison cannot be performed. In an image display apparatus which enables local dimming, this problem could also occur when a screen is divided into a plurality of display regions and different images displayed in different display regions are compared.